Not Your Average Case
by bandnerd21
Summary: There's something unusual about Detective Beckett that draws Castle to want to figure her out. But what is there to figure out? And who are those FBI agents who obviously aren't FBI? T to be safe, eventual Caskett, not sure how much Winchester there'll be, but we shall see.
1. Prologue

So, this is a thing... I was just saddened by the lack of Supernatural/Castle crossovers and after watching the episode of Castle, "Demons," I had the inspiration for this thing. I'm not sure how long it'll be or anything, but the Winchesters will eventually show up. This is, of course, AU for Castle, and I haven't decided where in the spn timeline this will be. Enjoy!

disclaimer: I obviously don't own either of these shows.

* * *

A nineteen-year-old Kate Beckett watched in horror as her mother fell, blood pooling around her body as the killer added a few more expertly-placed stabs in her already lifeless torso.

She had run into her mom on the way home from the grocery store in front of the dark alley. When a rustling was heard, Johanna pushed Kate into the shadows and whispered for her to stay quiet. It took everything she had not to cry out, but, as it were, she let out a soft, choked sob as Johanna's body hit the pavement.

The man leaned down to clean his knife on his pants before hearing Kate's voice, standing up, and smiling. Kate held her breath, but it was too late. The man was advancing.

Nearby, a cloud of buzzing black filth floated in the air, watching the scene with one part interest and one part mild worry. She wasn't sure what it was about this family, but she had carefully watched each family member for a few weeks and had grown quite attached to each one. Johanna Beckett's death angered her, and she felt an odd need to protect the young woman cowering in a corner. She hasn't determined the reason why yet. Perhaps it was that she had grown tired of destruction and longed for a taste of that long-forgotten humanity. She had, after all, lent a hand in the crucifixion of Christ, caused the fall of Pompeii... Oh yes, they were all her. Ordered by a higher-up, but it was her expertise that carried out the plan to bloody perfection.

She was jarred from her thoughts by the young Beckett's screams as the man stabbed her once from an angle that, the demoness calculated, would cause her to bleed out slowly. Quickly, the black cloud found her way into Kate's body, encountering her fading soul. She concentrated on healing the wound, but she was too late. The young Beckett was fading quickly. _I will spend your life as you would have. What do you want your life to be?_

Kate's last thought before fading into nothingness was _avenge my mother. Find her killer, put him behind bars. And stand up for the voiceless. Just like she did. _

So the new Kate stood and ran as quickly as her legs would carry her away from the alleyway, making certain anything tying her to the scene was cleared away. Kate Beckett never saw her mother die. She didn't know her mother was dead until she saw Detective Raglan at her house with the news. She cried with Kate's father, and she didn't even feel out of place. Slowly, she absorbed the memories Kate held dear and, without realizing it, became Kate. She closely followed each step of the case, grimacing when the detectives on the case overlooked obvious things, rules the case a mugging, and shoved it away in the back of a filing cabinet that wasn't even alphabetized. She resolved to become a detective. That was the best way to honor Kate's dying wish. She would become a homicide detective, speak for the voiceless, and solve Johanna's brutal murder.

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Hope you liked it! And, as goes for every story on here, please review. Reviews are like lifeblood. They make me eternally happy.


	2. Chapter 1

"So get this," Sam Winchester began, turning his laptop around to face his brother, who happened to be stuffing his face with a hamburger. Dean wiped his hand across his mouth before taking a closer look at the laptop. On it was an article about a young woman, a new homicide detective. According to the article, she's the youngest woman to ever reach that rank.

Dean shrugged. "So?"

"So, I found this published the day after a series of freak thunderstorms in New York."

"So what, you thinking demon-deal?"

"Amazing achievement the day after strange weather patterns? I figure we should check it out."

Dean looked at the young woman again. "She's pretty hot."

Sam rolled his eyes. "I suppose a trip to the coast won't hurt. But I say just scout it out. Not much we can do until her time's up."

Dean grunted in agreement before going back to his burger.

That afternoon, the brothers took off for New York. They started in Minnesota and had about a 19-hour drive ahead of them. With Dean's driving skills, they made it to New York City in about 15 hours. They checked into a seedy motel and immediately crashed.

* * *

Detective Beckett stood in front of the mirror and toweled her hair dry the night after her first murder scene as a detective. She had known right off when those so-called "FBI Agents" with the names of classic rock idols showed up and started snooping around her first official crime scene that there was something not quite right. Something about them screamed unprofessional.

There was also something else about them. Something that her demonic side, dormant for years now, sensed as predatory and reacted to. She saw her eyes turn a deep purple in the mirror, and she let them stay that way. She was pissed off, tired, and her mother's case was at a stand still for her. Her eyes hadn't changed since the night of Johanna Beckett's murder, and she was just so damn tired. She put her hair in a ponytail and walked into the kitchen to pour herself a stiff drink and read another chapter in Rick Castle's latest novel. The intrigue and mystery comforted her as she constantly tried to solve the unsolvable. She put her mother's files aside for the night and settled into the couch.

She thoughtfully leafed through the crisp pages, sipping her drink, until a crash from outside the door of her apartment caused her to pause, put her book and drink down, phase her eyes back to their brown human form, and walk warily to the door. She pulled her gun from the kitchen counter and turned the safety off. Beckett took a look through the peephole of her door, saw no sign of anyone in the hallway, steadied her gun, and threw open the door, glancing right then left. Then she looked to the right again as a shock of brown hair disappeared from view. Kate grimaced; maybe she was a little paranoid from her first day on the job, but she had a strange feeling those FBI guys were following her.

Beckett sighed, backed through the door, closed, locked, and bolted it, and put her gun back on the counter before returning to her book and drink, trying to shake off the paranoid feeling. It didn't help that those agents were actually hunters. She sighed and started reading. Not five minutes later, there was a knock at her door. Inside, Beckett set down the book-halfway through the chapter-and got up to answer the door. The hunters were standing in front of her, and the tall one smiled tightly.

She returned the tense smile. "Agent Hammel, Agent Scott. What can I do for you?"

"We just wanted to go over the case with you more thoroughly." Sam's tight smile grew tighter, if that was possible

She leaned back and checked the clock on her stove. "It's ten thirty, Agent. Surely you could've waited until the morning?"

"Not this, Detective," Dean interjected quickly.

She shrugged. "In that case, by all means, do come in."


	3. Chapter 2

**A/N: I know many of you like to know timelines for fics, as do I, so I thought I should tell you, the beginning of the Winchesters' involvement in this is somewhere in season 2. The Castle timeline is fairly obvious, though. For the sake of my sanity, the Castle cast is going to be a few years younger than the Supernatural cast, even though Jensen and Stana are the same age. There will be some skipping around, and a few chapters after this chapter will be a "10 years later" scene, which means it will be during a fictitious season 12 for Supernatural and an also fictitious season 6 of Castle.**

* * *

She showed the agents to her couch and offered them beers. She came back from the fridge, drinks in hand, and raised her eyebrows.

"So, what exactly did you want to discuss about the case, Agents?"

Dean cleared his throat. "We were wondering if there'd been anything out of the ordinary going on around the time you were promoted."

She took a sip of her drink. _Keep playing dumb or be frank._ She shrugged. "This is New York. Plenty of strange happens here on a daily basis."

The hunters exchanged glances before Sam coughed and took a sip of his beer.

"We were wondering more nature. Strange electrical storms and the like."

She put on the thoughtful face she knew they wanted. She knew what they were implying; they thought Detective Beckett had made a deal with a demon for some unknown reason. If only they knew... "No, I can't say there were." The two men exchanged wary glances, and Beckett sighed. "Now, though this wasn't a lovely visit, if you have no questions about the case, I would like to get some sleep tonight."

The brothers nodded and stood.

She showed them out the door, breathing a sigh of relief when she closed it. Tugging lightly on her ponytail, she returned to following the adventures of Derek Storm.

* * *

"I think she's hiding something," Dean stated the second the door had closed.

"What makes you say that?"

"I don't know. She just looked... Shifty."

"Well, we have to hang around until the end of this case, so we might as well keep an eye on her."

Dean nodded and finished off his beer. "Well, I'd say it's time to hit the hay."

"Agreed."

The two headed off to the seedy motel and didn't stir until they had to go to the station the next morning.

* * *

Detective Beckett stood staring at the murder board, sipping pensively on her coffee. Something just wasn't adding up. She set her coffee down and, for the umpteenth time that day, ran her hands through her hair in frustration.

"Everything all right," Esposito asked on his way back to his desk from the coffee machine.

"Yeah, just... Something about this case isn't making sense."

Javier leant against the desk next to her. "What is it?"

She showed him the facts and figures, explaining how it all should lay out, and then tearing all the "shoulds" down because all of the suspects had alibis.

Esposito frowned before asking, "have you asked the gardener?"

She glanced at him. "The records don't show they had a gardener."

He passed her a file. "He wasn't exactly hired help. Needed community service, and I guess the victim was unfortunate enough to offer him a place as the gardener. Want me to pick him up?"

"Yeah, that'd be great. Why didn't anyone give me this information beforehand?"

"It just came up this morning. You looked like you needed someone to rant to for a bit, so I waited."

She nodded, and he walked off to find Ryan.

She let out a sigh, scratching her head. Hopefully not all cases would be like that one.

As if on cue, the hunters walked through the door and headed straight for her. She smiled tightly and closed the distance with a few short steps.

"We just got our guy. Gardener of the victim. Guess it's not one of yours."

The taller one nodded shortly. "Seems we better get off your case then."

"Seems like it. It was a pleasure working with you two." She held out her hand, and the two shook it, smiled, and walked off. She whispered a quiet thank you to the empty air around her before going to get her second coffee of the day. She took it back to her desk and opened the file. Quite the record on this one. _Attempted assault, so he's got a temper, multiple counts of robbery... At least this one's straight forward. Clear motive from a clear killer. _Her thoughts drifted to her mother's case before she shoved them to the back of her mind, shaking her head angrily. She needed to get over this.

* * *

Dean grumbled the entire way out of the station. It was only after they had climbed into the impala that he hit the steering wheel, saying, "That was way too convenient for my taste."

Sam shrugged. "Honestly, if she doesn't want to say anything, we can't make her. We'll just have to wait around for everything to fall to shit."

"Why do you think she made the deal, though?"

"When we were in her apartment, did you notice the case file she tried to hide under that Richard Castle book?"

Dean thought for a moment before nodding, and realization dawned on his face. "It was a murder file for Johanna Beckett, wasn't it? Mom or sister, maybe?"

"The date on the file makes me think mother. My guess is she made a deal to be able to solve that murder-they put it down as a random mugging, but she didn't seem the type to believe that."

Dean nodded and threw his baby into gear, speeding off away from the station to get some food. "So what, we just come back in ten years?"

Sam shrugged, and the hunters began the drive out of New York.

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**A/N: So, I don't like throwing a lot of these author's notes around, but I did want to spend a sentence or two begging for reviews. So, please review. Reviews are lifeblood and make me quite incredibly happy.**

**There, that wasn't so bad, was it? And just click that happy little button ****_please_****!**


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